This application is based on an application Nos. 10-127989, 10-153323, 10-157295, 10-252548, and 11-5016 filed in Japan, the contents of which are hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a plasma display panel used for a display device, and especially relates to a plasma display panel including an improved dielectric glass layer.
(2) Description of the Prior Art
Recently, expectations for a high-definition TV and a large-screen TV have been raised. For such a TV, a CRT display, a liquid crystal display, or a plasma display panel has been conventionally used as a display device. A CRT display is superior to a plasma display panel and a liquid crystal display in resolution and image quality. A CRT display, however, is not suitable for a large screen that measures more than 40 inches because the depth. dimension and the weight are too large. A liquid crystal display is superior in consuming a relatively low power and requiring a relatively low voltage. A liquid crystal display, however, has disadvantages of a limited screen size and viewing angle. On the other hand, a plasma display panel realizes a large screen. Screens that measure in the 40 inches have been developed using plasma display panels (described in xe2x80x9cKino Zairyo (Functional Materials)xe2x80x9d (Vol. 16, No. 2, February issue, 1996, p7), for instance).
FIG. 13 is a perspective view of the essential part of a conventional ac plasma display panel. In FIG. 13, a reference number 131 refers to a front glass substrate made of borosilicate sodium glass. On the surface of the front glass substrate, display electrodes 132 are formed. The display electrodes 132 are covered by a dielectric glass layer 133. The surface of the dielectric glass layer 133 is covered by a magnesium oxide (MgO) dielectric protective layer 134. The dielectric glass layer is formed using a glass powder the particle diameter of which ranges from 2 to 15 xcexcm on average.
A reference number 135 refers to a back glass substrate. On the surface of the back glass substrate 135, address electrodes 136 are formed. The address electrodes 135 are covered by a dielectric glass layer 137. On the surface of the dielectric glass layer 137, walls 138 and phosphor layers 139 are formed. Between the walls 138, discharge spaces 140 are formed. The discharge spaces 140 are filled with discharge gas.
A full-specification, high-definition TV is expected to realize the pixel level given below. The number of pixels is 1920xc3x971125. The dot pitch is 0.15 mm xc3x970.48 mm for a screen that measures around 42 inches. The area of one cell is as small as 0.072 mm2. The area is {fraction (1/7+L )} to {fraction (1/8+L )} compared with a 42-inch, high-definition TV according to a conventional NTSC (National Television System Committee) (the number of pixels is 640xc3x97480, the dot pitch is 0.43 mmxc3x971.29 mm, and the area of one cell is 0.55 mm2).
As a result, the intensity of the panel decreases for the full-specification, high-definition TV (described in xe2x80x9cDisupurei Ando Imeijingu (Display and Imaging)xe2x80x9d Vol. 6, 1992, p70, for example).
In addition, not only the distance between the discharge electrodes is shorter, but also the discharge space is smaller for the full-specification, high-definition TV. As a result, when the plasma display panel gains the same capacity as a capacitor, it is necessary to set the thickness of the dielectric glass layers 133 and 137 to be smaller than in a conventional one.
Here, the explanation of three methods of forming a dielectric glass layer will be given below.
In the first method, a glass paste is made of a glass powder the particle diameter and the softening point of which ranges from 2 to 15 xcexcm on average and from 550 to 600xc2x0 C., and a solvent such as terpineol including ethyl cellulose and butyl carbitol acetate using a trifurcated roll. The glass paste is printed on the front glass substrate according to a screen printing method (the glass paste is adjusted so that the viscosity is 50,000 to 100,000 cp, which is suitable for the screen printing method). The printed glass paste is dried, and undergoes sintering at a temperature around the softening point of the glass powder (550 to 600xc2x0 C.), forming a dielectric glass layer.
In the first method, the melted glass rarely reacts to the electrode made of Ag, ITO, Cr-Cu-Cr, or the like since the glass paste undergoes sintering at a temperature around the glass powder softening point and the glass is inert, i.e., the glass does not flow well. As a result, the resistance of the electrode does not increase, the electrode ingredients do not dispersed in or not color the glass, and a dielectric glass layer is formed with one firing. On the other hand, the glass paste does not flow well since the particle diameter of the glass powder ranges from 2 to 15 xcexcm on average and the glass paste is fired at a temperature around the softening point of the glass powder, and the mesh pattern of the screen remains in this method. As a result, the surface of the formed dielectric glass layer is rough (the surface roughness is 4 to 6 xcexcm), and visible light is scattered on the coarse surface. In other words, the dielectric glass layer is a ground glass and the transmittance is relatively low. In addition, bubbles and pinholes appear in the formed dielectric glass layer, so that the voltage endurance of the dielectric glass layer is decreased. Here, the voltage endurance means the limitation of the insulation effect of a dielectric glass layer when a voltage is applied to the dielectric glass layer.
In the second method, a glass paste (the viscosity is 35,000 to 50,000 cp (centipoise)) is made using a low-melting lead glass powder (the proportion of PbO is about 75%) the particle diameter and the softening point of which ranges from 2 to 15 xcexcm on average and from 450 to 500xc2x0 C. The glass paste is printed on the front glass substrate according to a screen printing method and dried. The dried glass paste undergoes sintering at a temperature about 100xc2x0 C. higher than the softening point of the glass powder, i.e., at 550 to 600xc2x0 C., forming a dielectric glass layer. In the second method, the surface of the formed dielectric glass layer is smooth (surface roughness is about 2 xcexcm) since the sintering temperature is considerably higher than the softening point and the glass paste flows well. In addition, a dielectric glass layer is formed with one sintering.
On the other hand, the melted glass reacts to the electrode made of Ag, ITO, Cr-Cu-Cr, or the like since the glass paste is activated and flows well. As a result, the resistance of the electrode increases and the dielectric glass layer is colored. In addition, large bubbles are likely to appear in the dielectric glass layer as a result of the reaction to the electrode.
The third method is the combination of the first and second methods (refers to Japanese Laid-Open Patent Application Nos. 7-105855 and 9-50769). In the third method, a glass paste is made of a glass powder the particle diameter and the softening point of which ranges from 2 to 15 xcexcm on average and from 550 to 600xc2x0 C. The glass paste is printed on the front glass substrate according to the screen printing method. The printed glass paste is dried, and undergoes sintering at a temperature around the softening point, forming a dielectric glass layer. On the formed dielectric glass layer, another dielectric glass layer is further formed. A glass paste is made of a glass powder the particle diameter and the softening point of which ranges from 2 to 15 xcexcm on average and from 450 to 500xc2x0 C. The second glass paste is printed on the previously formed dielectric glass layer according to the screen printing method. The printed second glass paste is dried, and undergoes sintering at a temperature about 100xc2x0 C. higher than the softening point, i.e., at 550 to 600xc2x0 C., forming the second dielectric glass layer.
Due to the bilevel structure, the melted glass rarely reacts to the electrode and the surface of the dielectric glass layer is smooth, resulting in an improved transmittance of visible light and endurance to voltage. At the same time, however, the method of forming the dielectric glass layer is complicated and a thinner dielectric glass layer, which is necessary to improve the intensity, is difficult to form. In addition, the visible light transmittance is not improved so much since bubbles appear in the first formed dielectric glass layer.
It is accordingly an object of the present invention to provide a reliable, high-intensity plasma display panel in which the visible light transmittance is high even when the plasma display has a fine cell structure since the problems of low visible light transmittance and low voltage endurance are solved. The above-mentioned object may be achieved by the manufacturing method of plasma display given below.
In the manufacturing method of plasma display, a glass paste including a glass powder the average particle of which is 0.1 to 1.5 xcexcm and the maximum particle diameter of which is equal to or smaller than three times the average particle diameter is printed on the front glass substrate or the back glass substrate on which electrodes have been formed according to a screen printing method, a die coating method, a spray coating method, a spin coating method, and a blade coating method. Then, the glass powder in the printed glass paste undergoes sintering, forming a dielectric protective layer.
The object of the present invention may be realized since a dielectric glass layer having a relatively smooth surface and including a minimum amount of bubbles is formed using the glass powder that has been described.